Retrofit2源码解析02-创建Call

深碍√TFBOYSˉ_ 2023-10-17 16:44 59阅读 0赞

我们在获得了Retrofit对象后，通过create方法可以获得生成接口的代理对象。

ApiService service = retrofit.create(ApiService.class);

来看create方法

public <T> T create(final Class<T> service) {
        
        Utils.validateServiceInterface(service);
        //如果validateEagerly为true，则进行预处理，提前创造一些MethodHandler对象在缓存中，默认为true。
        if (validateEagerly) {
        
          eagerlyValidateMethods(service);
        }
        //返回一个动态代理对象
        return (T) Proxy.newProxyInstance(service.getClassLoader(), new Class<?>[] {
         service },
            new InvocationHandler() {
        
              private final Platform platform = Platform.get();
    
              @Override public Object invoke(Object proxy, Method method, Object... args)
                  throws Throwable {
        
                // If the method is a method from Object then defer to normal invocation.
                if (method.getDeclaringClass() == Object.class) {
        
                  return method.invoke(this, args);
                }
                if (platform.isDefaultMethod(method)) {
        
                  return platform.invokeDefaultMethod(method, service, proxy, args);
                }
                ServiceMethod serviceMethod = loadServiceMethod(method);
                OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
                return serviceMethod.callAdapter.adapt(okHttpCall);
              }
            });
        }

首先调用了Utils.validateServiceInterface(service);

static <T> void validateServiceInterface(Class<T> service) {
        
            if (!service.isInterface()) {
        
              throw new IllegalArgumentException("API declarations must be interfaces.");
            }
            // Prevent API interfaces from extending other interfaces. This not only avoids a bug in
            // Android (http://b.android.com/58753) but it forces composition of API declarations which is
            // the recommended pattern.
            if (service.getInterfaces().length > 0) {
        
              throw new IllegalArgumentException("API interfaces must not extend other interfaces.");
            }
        }

可以看到，如果传入的Class不是接口，或者这个Class有继承自其它接口，则抛出异常。

再来看create方法中的`Proxy.newProxyInstance`，可以看到，新new了一个invocationHandler。主要来看InvocationHandler

new InvocationHandler() {
        
          private final Platform platform = Platform.get();
        
          @Override public Object invoke(Object proxy, Method method, Object... args)
              throws Throwable {
        
            //method.getDeclaringClass() 返回方法定义所在的类
            //如果method定义所在的类就是object，直接通过反射调用
            if (method.getDeclaringClass() == Object.class) {
        
              return method.invoke(this, args);
            }
            //只有Java8实现了这个方法，其它平台都是返回false
            if (platform.isDefaultMethod(method)) {
        
              return platform.invokeDefaultMethod(method, service, proxy, args);
            }
            //加载ServiceMethod (优先从缓存中获取)
            ServiceMethod serviceMethod = loadServiceMethod(method);
            //传入serviceMethod和args，创建OkHttpCall
            OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
            //会创建ExecutorCallbackCall，并传入OkHttpCall
            return serviceMethod.callAdapter.adapt(okHttpCall);
          }
        });

如果method定义所在的类就是object，直接通过反射进行调用。如果不是，则会通过loadServiceMethod加载serviceMethod。获取ServiceMethod之后，创建一个OkHttpCall，并传入到serviceMethod的callAdapter.adapt()。

先来看下loadServiceMethod，这个会优先从缓存中获取，如果获取为null，则新创建一个serviceMethod，并存储到缓存中。

ServiceMethod loadServiceMethod(Method method) {
        
            ServiceMethod result;
            synchronized (serviceMethodCache) {
        
              //优先从缓存中获取
              result = serviceMethodCache.get(method);
              if (result == null) {
        
                result = new ServiceMethod.Builder(this, method).build();
                serviceMethodCache.put(method, result);
              }
            }
            return result;
        }

来看下serviceMethod是怎么被创建的。可以看到，

1.  首先遍历获得合适的CallAdapter和responseConverter。
2.  遍历method的注解，对请求方式(GET,POST)进行解析
3.  遍历method传入参数的注解(@Query、@Part)，进行解析

#####  #####

public ServiceMethod build() {
        
          //会遍历adapterFactories列表中存储的CallAdapter.Factory，通过get方法返回CallAdapter对象，判断并返回一个合适的CallAdapter。
          //(也就是通过addCallAdapterFactory进行添加的列表)
          callAdapter = createCallAdapter();
          //得到返回数据的真实类型
          responseType = callAdapter.responseType();
          //...
          //遍历converterFactories列表中存储的Converter.Factory，通过responseBodyConverter方法返回Converter方法，判断并返回一个合适的Converter用来转换对象。  
          //(也就是通过addConverterFactory进行添加的列表)
          responseConverter = createResponseConverter();
          //遍历method的注解，调用parseMethodAnnotation对请求方式(比如GET、POST)和请求地址进行解析
          for (Annotation annotation : methodAnnotations) {
        
            parseMethodAnnotation(annotation);
          }
          //...
          int parameterCount = parameterAnnotationsArray.length;
          parameterHandlers = new ParameterHandler<?>[parameterCount];
          for (int p = 0; p < parameterCount; p++) {
        
            Type parameterType = parameterTypes[p];
            if (Utils.hasUnresolvableType(parameterType)) {
        
              throw parameterError(p, "Parameter type must not include a type variable or wildcard: %s",
                  parameterType);
            }
        
            //对方法中的参数注解进行解析 (比如@Query、@Part)  
            Annotation[] parameterAnnotations = parameterAnnotationsArray[p];
            if (parameterAnnotations == null) {
        
              throw parameterError(p, "No Retrofit annotation found.");
            }
        
            parameterHandlers[p] = parseParameter(p, parameterType, parameterAnnotations);
          }
          //...
          return new ServiceMethod<>(this);
        }

createCallAdapter和createResponseConverter的代码逻辑类似，以createCallAdapter为例。  
首先会获取Type和注解数组，然后调用callAdapter，最终，通过Type和注解数组获取相应的CallAdapter，并返回。

private CallAdapter<?> createCallAdapter() {
        
          //获取method的Type (java反射)
          Type returnType = method.getGenericReturnType();
          //...
          //获取method的注解数组
          Annotation[] annotations = method.getAnnotations();
          try {
        
            return retrofit.callAdapter(returnType, annotations);
          } catch (RuntimeException e) {
        
            throw methodError(e, "Unable to create call adapter for %s", returnType);
          }
        }
        
        public CallAdapter<?> callAdapter(Type returnType, Annotation[] annotations) {
        
            return nextCallAdapter(null, returnType, annotations);
        }
        
        public CallAdapter<?> nextCallAdapter(CallAdapter.Factory skipPast, Type returnType,
          Annotation[] annotations) {
        
            checkNotNull(returnType, "returnType == null");
            checkNotNull(annotations, "annotations == null");
            
            int start = adapterFactories.indexOf(skipPast) + 1;
            for (int i = start, count = adapterFactories.size(); i < count; i++) {
        
                CallAdapter<?> adapter = adapterFactories.get(i).get(returnType, annotations, this);
                if (adapter != null) {
        
                    return adapter;
                }
            }
        }

至此，ServiceMethod就初始化完毕了。

再来看之前InvocationHandler中的serviceMethod.callAdapter.adapt()

ServiceMethod serviceMethod = loadServiceMethod(method);
        OkHttpCall okHttpCall = new OkHttpCall<>(serviceMethod, args);
        return serviceMethod.callAdapter.adapt(okHttpCall);

调用loadServiceMethod之后，会先创建一个OkHttpCall，这个的构造方法里面只是进行了赋值操作。

OkHttpCall(ServiceMethod<T> serviceMethod, Object[] args) {
        
            this.serviceMethod = serviceMethod;
            this.args = args;
        }

之后，会调用`serviceMethod.callAdapter.adapt()`。  
我们在前一篇文章里面讲过，serviceMethod.callAdapter是用作将OkHttp的call对象转化为需要的对象，比如默认的ExecutorCallAdapterFactory，又比如转化为RxJava的Observable的RxJavaCallAdapterFactory。

这里来看ExecutorCallAdapter.Factory

@Override public <R> Call<R> adapt(Call<R> call) {
        
            return new ExecutorCallbackCall<>(callbackExecutor, call);
       }

可以看到新new了一个ExecutorCallbackCall

static final class ExecutorCallbackCall<T> implements Call<T> {
        
            final Executor callbackExecutor;
            final Call<T> delegate;
            
            ExecutorCallbackCall(Executor callbackExecutor, Call<T> delegate) {
        
              this.callbackExecutor = callbackExecutor;
              this.delegate = delegate;
            }
            
            @Override public void enqueue(final Callback<T> callback) {
        
              if (callback == null) throw new NullPointerException("callback == null");
            
              delegate.enqueue(new Callback<T>() {
        
                @Override public void onResponse(Call<T> call, final Response<T> response) {
        
                  callbackExecutor.execute(new Runnable() {
        
                    @Override public void run() {
        
                      if (delegate.isCanceled()) {
        
                        // Emulate OkHttp's behavior of throwing/delivering an IOException on cancellation.
                        callback.onFailure(ExecutorCallbackCall.this, new IOException("Canceled"));
                      } else {
        
                        callback.onResponse(ExecutorCallbackCall.this, response);
                      }
                    }
                  });
                }
            
                @Override public void onFailure(Call<T> call, final Throwable t) {
        
                  callbackExecutor.execute(new Runnable() {
        
                    @Override public void run() {
        
                      callback.onFailure(ExecutorCallbackCall.this, t);
                    }
                  });
                }
              });
            }  
            
            //...
        }

可以看到ExecutorCallbackCall是对Call的封装，主要通过callbackExecutor(这个是在Retroft构建的时候创建的，android平台的默认的是MainThreadexecutor，主要是将runnable回调到主线程执行)，在主线程中执行runnable。

而ExecutorCallbackCall继承自Retort的Call，enqueue中，会通过delegate.enqueue进行调用，这里的delegate是OkHttpCall。回调结果中，会去调用到主线程，然后通过callback进行回调。

#### 总结 ####

retrofit.create方法的内部会创建一个动态代理对象并返回，真正的执行者是这个动态代理对象。

1.  如果method定义所在的类就是object，直接通过反射调用。
2.  加载ServiceMethod(优先通过缓存)  
    3. 会遍历并选用合适的callAdapter和responseConverter  
    4. callAdapter:将Call对象转化的合适执行类，比如RxJavaCallAdapterFactory  
    5. responseConverter: 转化数据对象，比如GsonConverterFactory  
    3. 会解析method和method传参的注解，比如@Get、@Query
3.  获得serviceMethod后，会创建OkHttpCall
4.  调用serviceMethod.calladapter.adapt(okhttpcall)，并返回Retroft的Call方法。

#### serviceMethod.calladapter.adapt深入 ####

这里的calladapter可以自己设置，比如RxJavaCallAdapterFactory，这里以默认的ExecutorCallAdapterFactory为例。

当调用Retrot的Call的enqueue的时候，交由delegate这个okhttp代理对象执行，并在返回结果中，先回调到主线程，执行callback的相关方法。

#### 其他 ####

> 文中的Retrfot版本为Retrfot2.1.0